Page 8 - Offshore Well Construction

CHAPTER 4
Drilling and Completion Fluids
Petroleum Extension-The University of Texas at Austin

FUNCTIONS OF A DRILLING FLUID
The primary functions of a drilling fluid are:
• Well control
• Maintain hole stability
• Hole cleaning
• Transmit hydraulic horsepower to the bit
• Formation evaluation
These functions are achieved by careful selection of the drilling fluid and maintenance of its properties.

Additional functions of a drilling fluid are:
• Suspend cuttings and weighting agent while the fluid is static; e.g., connections
• Release entrained cuttings at surface
• Cool and lubricate the bit and drill string
• Create a thin, impermeable filter cake to reduce fluid invasion
• Support tubulars through buoyancy effect
• Prevent and control corrosion of drill string, etc.

TYPES OF DRILLING FLUID
There are three main types of drilling fluid, distinguished by their base fluid formulation.

Air/Gas
Used for drilling hard, dry formations or to combat lost circulation. Rarely used offshore, except for underbalanced
or coiled-tubing drilling.

Water-Based Mud

The main types of water-based mud are
• Nondispersed
• Dispersed
• Calcium treated
• Polymer
• Low solids
• Salt water

Nondispersed Muds
Generally includes lightly treated, low-weight muds, and spud muds. No thinners added. Usually top hole and shallow
well applications.

Dispersed Muds
With increasing depths and mud weights, mud formulations require dispersant additives (lignosulphonates, lignites,
tannins) to cancel the interparticle attractive forces that create viscosity in water-based mud. This effectively extends
the use of the mud system until it has to be replaced.

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